[cairo] Tracking down stale references to entries in the scaled font map

[Zack Weinberg]

I'm trying to solve Mozilla bug 503924, where various of our attempts
to clean up library state on exit (so we can detect memory leaks) cause
crashes deep within pango and/or cairo.  Right now we're crashing
inside cairo_debug_reset_static_data because there are two live fonts
still in cairo_scaled_font_map when _cairo_hash_table_destroy is called
on it.  These fonts are, unfortunately, the fontconfig defaults for
serif and sans, so there is no obvious place to begin looking for the
stale references.

Can anyone give some tips on how to track down where these stale
references might be held?

Thanks,
zw